Engine exhaust emission control regulations are becoming increasingly demanding throughout the world. In addition to lowering permissible emission levels for hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM), on-board measures for diagnosing the operability of emission-related engine operations and aftertreatment devices have been mandated for both spark and compression ignition engines. While advances in engine designs have helped minimize pollutant generation within the combustion process, improvements in catalyst performance such as in light-off and thermal durability have aided in the lowering of tailpipe emission levels.
As a result, in most cases the most significant breakthrough of regulated emissions to the atmosphere occurs during the period when the emission catalysts are warming to temperatures where efficient conversion can take place. Developers of future system designs which will employ the most sophisticated engine controls to facilitate catalyst heat-up are searching for additional measures to further reduce emissions during what has come to be known as the cold start of the engine.
One measure for addressing HC emissions during cold start is by employing an exhaust aftertreatment component referred to as a hydrocarbon adsorber. HC adsorbers are devices which are designed to remove hydrocarbon molecules from a relatively cool exhaust stream, at a temperature below the light-off temperature of oxidation catalysts, and trap or store the HC until the exhaust gas temperature rises to a point where downstream catalytic sites are active. With the increasing use of alcohols in engine fuels, and specifically with an increase in the ethanol content of many fuels targeted for use with conventional gasoline fueled engines, the emission of alcohols and the products generated from the partial combustion of alcohols is becoming a more important problem during engine cold start as well. The devices known as HC adsorbers can also be effective for the storage of alcohols and other oxygenates during cold start. For this reason, in the description below, when the term HC is utilized it should be understood that related phenomena associated with oxygenates apply, and the scope of the invention includes the treatment of such species as well.
Many high surface area materials such as the support oxides commonly used in exhaust catalysts are capable of storing some HC at low temperatures which can subsequently be released as the exhaust gas temperature increases. Even the porous ceramic substrates composed of cordierite or mullite have some capacity for low temperature HC storage. It has become known, however, that highly porous structures like the crystalline microporous silica-aluminas, also known as molecular sieves or zeolites, and related structural compositions, are able to store significant quantities of HC at low temperatures, and because of the strong physical interaction between the HC molecules and these materials, the onset of HC desorption occurs at higher temperatures. This, in turn, allows the associated downstream catalytic sites to heat up to higher temperatures and thereby increases the overall HC conversion efficiency of the system.
There are a wide variety of molecular sieve materials which may be particularly useful as HC adsorbents. These include aluminosilcate zeolites, metal-substituted aluminosilicate zeolites, and aluminophosphate materials which include AlPO, MeAlPO, SAPO, and MeAPSO compositions. Additionally, materials comprising a range of crystallographic structures with Framework Type Codes consisting of BEA, FAU, MOR, MFI, FER, and CHA, among others. A key attribute to any of these is the presence of sufficient thermal durability to withstand the needs of the specific configuration's most severe exhaust temperature.
Because the types of HC or oxygenate molecules in engine exhaust range from single carbon species up to molecules with 8 carbon atoms and more, effective HC adsorbents must have the ability to capture a broad range of species ranging in physical size, shape, and volatility. While molecular sieves with larger pore and channel sizes such as those possessing the BEA, MOR, and FAU structures can easily accommodate larger molecules such as substituted benzenes, toluene, pentane, and hexane within their internal structures, thereby facilitating the storage of these molecules and delaying their release during the heat-up process accompanying cold start, smaller molecules such as ethane, propane, and ethanol are released from these structures at temperatures below their light-off temperatures over typical precious metal catalysts. As a result, larger pore molecular sieves when applied by themselves may be inadequate for the purpose of HC control during cold start. On the other hand, molecular sieves with smaller pore sizes such as those possessing the FER and CHA structures, while unable to accommodate larger HC and oxygenate molecules within their internal structures, are able to capture and store smaller molecules, thereby facilitating their combustion over sufficiently heated precious metal sites at a later stage in the heat-up process associated with engine cold start. Because of these differences in storage and release properties, mixtures of molecular sieve materials can be used advantageously within a HC adsorber formulation.
The simplest HC adsorbers comprise an HC storage component (adsorbent) such as a zeolitic material, a “honeycomb” ceramic or metallic substrate, and a binder to promote adhesion of the adsorbent to the substrate. Binders are well known in the art and any suitable binder can be used for purposes of the present invention. As with related exhaust emission catalysts, coated ceramic substrates are incorporated into a metal container, referred to as a converter, with an inlet and outlet allowing exhaust gas to be directed through the channels of the coated substrate.
As noted before, the fundamental principle of adsorber operation is low temperature HC storage and its subsequent thermal desorption and oxidation at an elevated temperature over downstream catalytic sites. In practice, these downstream sites may be located on a separate and distinct catalyzed substrate, or even within a separate, downstream catalytic converter.
More complex formulations for HC adsorbers are also well established in the art where the concept of downstream site may be extrapolated to adjacent catalytic site. In addition to the HC adsorbent, components with catalytic activity for HC and CO oxidation and NOx reduction, as well as materials with oxygen storage capacity (OSC), may be incorporated within the same device. Each of these components is well known for their contribution to the activity of state-of-the-art three-way catalysts (TWC). Catalytic components include precious metals such as platinum, palladium, and rhodium, typically supported on alumina or other metal oxides. Standard OSC components include materials comprising oxides of cerium and its combinations with zirconium, yttrium, neodymium, praseodymium, and lanthanum, among others. Architectured structures comprising layers of HC adsorbent and TWC supported on honeycomb substrates have also been utilized commercially as HC adsorbers.
In addition to variations in HC adsorber formulation, systems have also been employed on production vehicles in two distinct configurations, so called in-line and off-line designs. Examples of these configurations are shown in FIG. 1 and FIGS. 2, 2A and 2B. The in-line configuration offers advantages in cost and packaging, however, it also requires the HC adsorber to maintain performance following exposure to high temperature exhaust. The off-line system adds the complexity of a valved exhaust, but minimizes exposure of the adsorbent to elevated temperatures, while providing a mechanism by which HC can be isolated in an off-line adsorbent as a downstream catalyst is allowed to heat up to a more efficient operating temperature before the trapped HC is released for oxidation.
One of the challenges associated with the implementation of the in-line HC adsorber configuration is the degradation of the HC adsorbent upon exposure of the device to elevated exhaust temperatures which are common to operation of spark ignited engines under conditions of high load. As the adsorbent ages thermally, its surface area decreases. This can occur through conventional sintering processes, including the collapse of the crystalline microporous structures of molecular sieves and related materials. With the loss of surface area and associated porosity, the ability of the HC adsorber to perform its designed HC storage function is diminished, resulting in a decrease in the amount of HC which can be stored, as well as a lowering in the temperature range at which stored HC is thermally released from the adsorbent. In parallel, hydrothermal degradation of catalytic activity located either within the HC adsorber itself, or within a downstream catalyst can result in an increase in light-off for the oxidation of HC, as well as a loss in peak HC conversion efficiency. The combination of decrease in HC storage capacity, lowering of HC desorption temperature, and increase in HC oxidation light-off temperature results in an increase in unconverted HC being exhausted at the tailpipe during subsequent engine cold starts. Ultimately this decrease in system HC conversion efficiency can lead to a failure to meet legislated tailpipe emission levels.
Because of the system configuration, all exhaust flow passes through the in-line HC adsorber, regardless of the exhaust temperature. Upon prolonged exposure to exhaust flow at engine idle and other engine operating conditions where low exhaust temperatures persist, the HC adsorbent can accumulate HC which can subsequently transform by chemical reaction into tars, coke, and other compositions. These deposits can cover the adsorbent surface and foul its associated porosity. In this state the ability of the adsorbent to carry out its designed HC storage function is diminished which could ultimately result in unconverted HC being exhausted at the tailpipe. The activity of a fouled HC adsorber can be at least partially recovered by exposing the device to an exhaust flow of sufficient temperature. Often the process is more effective in the presence of oxygen such that the fouling deposits are combusted. Importantly, this process is exothermic and measures must be taken to limit the combustion rate during the “burn-out” process so that thermal damage of the adsorbent and any associated catalyst is minimized.
Because similar formulations can be utilized for different HC adsorber configurations, performance degradation mechanisms for off-line HC adsorbers are similar to those for in-line HC adsorbers. As noted previously, however, the off-line adsorber can be isolated and protected from the effects of high temperature exhaust.
A requirement of many automotive emission control systems is the capability to diagnose component and system changes which can result in the failure of the emission control system, and its associated vehicle or equipment, to comply with regulated exhaust emission levels. This general type of monitoring is referred to as an on-board diagnostic or OBD. Over the years, the requirements for OBD have expanded to include a range of engine-based components and emission catalysts.
For conventional TWCs, the development and incorporation of oxygen storage materials to meet the performance requirements of these types of catalytic components is well known. In this context, oxygen storage materials chemically react with oxygen present in the exhaust gas coming in contact with the catalyst when the oxygen storage material is at a sufficiently high temperature to render it active. The extent or amount of oxygen that reacts with the oxygen storage material is generally referred to as its oxygen storage capacity (OSC). OSC is known to vary with a number of properties including temperature, material composition, and state of aging. The response of an OSC material can be altered by the presence and absence of precious metals which often facilitate oxygen transfer reactions. Precious metals themselves, as well as certain base metals (Cu, Mn, Ni, Fe, Co, Cr, Mo, W, Ce, and V, among others) and their oxides, typically have an associated oxygen storage capacity of their own.
The oxygen storage property imparted by ceria and ceria-based mixed oxides often serves as the basis of an onboard diagnosis (OBD) signal which is utilized to assess the state of an emission catalyst, and through appropriate correlation, establish whether the catalyst is effective in meeting its application-specific performance requirements. The measurement of OSC is typically accomplished using a set of oxygen sensors, in conjunction with control of engine air-to-fuel ratio and catalyst temperature, and knowledge of exhaust flow rate through the system.
A simple switching time test consists of passing a fuel rich exhaust across the first oxygen sensor, through the test catalyst, and finally across the second oxygen sensor. Upon maintaining this flow for sufficient duration at sufficient temperature, any oxygen stored within the OSC of the catalyst will react completely away and signals from the first and second oxygen sensors will become equivalent and indicative of a fuel rich exhaust state. Once accomplished, the composition of the exhaust is adjusted in a stepped change from fuel rich to fuel lean (and oxygen rich). As the fuel lean exhaust passes across the first sensor, the signal of the first sensor responds to the step change in exhaust composition corresponding to the lean state. Because oxygen in the fuel lean exhaust is consumed by reaction with the oxygen storage material in the catalyst, it is only upon saturation of the catalyst OSC that fuel lean exhaust containing oxygen passes across the second sensor, ultimately yielding a signal indicative of the lean state at this sensor. The elapsed time between the sensing of the lean condition at the first and second sensors minus the time required for exhaust gas to flow from the first to second sensor is commonly referred to as the delay time. Similar although not identical behavior is observed whether the exhaust composition is switched from rich to lean or lean to rich. The corresponding delay times are characteristic of the OSC and state of the catalyst.
More complex measurements related to the response of oxygen storage materials in emission catalysts to changes in exhaust gas oxygen content can be made and used to assess catalyst state, and ultimately correlate it to one or more aspects of associated emission control performance. For OBD purposes, most often the correlation has been associated with TWC HC emission performance. Importantly, this correlation is with the catalytic activity for the reaction of HC.
Key to any OBD method based on oxygen storage capacity is a fast and reliable determination of exhaust gas composition. Measurements such as delay time tests can be accomplished without an exact measurement of oxygen concentration. It is well known in the art that simple heated exhaust gas oxygen (HEGO) sensors which are highly non-linear and only able to differentiate fuel rich or fuel lean exhaust compositions are sufficient for these types of measurements. Importantly, HEGO sensors are known to be both reliable and inexpensive.
In recent years, more sophisticated engine control strategies have dictated the introduction of universal exhaust gas oxygen (UEGO) sensors. UEGO sensors provide signals which are proportional to air to fuel ratio. As a result, oxygen concentrations can be estimated directly and monitored continuously. Unfortunately, UEGO sensors are more susceptible to drift than their HEGO counterparts, as well as more expensive.
Thermocouples or thermistors have been successfully applied for the OBD of diesel oxidation catalysts which provide HC emission control, but also serve as a heat-up catalyst for the combustion cleaning of particulate filters, or for a similar role in the desulfurization of NOx adsorber catalysts. In these cases, because large exotherms are routinely generated across these catalysts, temperature measurement is sufficiently accurate to be an effective signal for OBD.
At the present time, the direct, fast, and continuous measurement of hydrocarbons for engine control and system diagnostic purposes has not been introduced for mass production vehicles. Technologies capable of these measurements are typically either too large for vehicle packaging, or simply too expensive. As a result, HC performance of vehicle emission catalysts continues to be inferred rather than directly measured for OBD purposes.
As noted previously, a variety of HC adsorber designs exist, some of which store and release HC, others which additionally react with HC. As a result, the successful monitoring of these functions for OBD purposes must account for these differences. U.S. Pat. No. 5,524,433 claims the effective monitoring of hydrocarbon trapping devices by the combination of exhaust flow sensing and HC concentration determination. During a storage and release sequence in a bypass HC adsorber system, the aggregate amount of HC removed from the exhaust during start-up conditions could be determined directly with a set of HC sensors, or indirectly using a set of UEGO sensors. In either case, the sensors are positioned upstream and downstream of the HC adsorber and difference determinations made. The approach has the advantage of direct measurement, however, it comes at the cost of incorporating a pair of expensive sensors.
U.S. Pat. No. 5,765,369 teaches a similar approach for monitoring the effectiveness of a bypass HC adsorber system. Here the measurement of a physical value in the exhaust gas which could be compared to a predetermined physical value is claimed, where said physical value includes gas temperature and toxic component concentration. Again, control and monitoring of the exhaust purification system is complicated and expensive because of the need to make accurate difference determinations.
Similar approaches are claimed in U.S. Pat. No. 6,145,304 where the measurement of air-fuel ratio is specified, and in U.S. Pat. No. 6,532,793 where the detection of at least one gas substance selected from the group consisting of hydrocarbons, carbon monoxide, and nitrogen oxides is specified.
A different approach for controlling and diagnosing the operation of a bypass hydrocarbon adsorber system is claimed in U.S. Pat. No. 6,378,298. In this system, exhaust gas feeding the adsorbent bed is cooled. Temperature sensors provide a signal used to control flow through the adsorbent, and a signal from a pressure sensor is used to confirm that appropriate flow through the adsorbent bed occurs. Again, however, signals from a set of air/fuel sensors are utilized to determine the effectiveness of the adsorbent by difference calculation as proposed in U.S. Pat. No. 5,524,433.
US 2007/0051094 A1 discloses an HC adsorbing material based on Ce exchanged zeolite which exhibits no OSC signal. This is attributed to the ionic state of the Ce. An OSC signal develops when the zeolite structure collapses, Ce is released, and bulk CeO2 forms. This method is capable of detecting a catastrophic failure of the zeolitic structure which results in an associated loss of HC storage capacity. Conversely, it is unable to detect the loss of HC storage capacity associated with the reversible fouling of the zeolite pores.
The challenge associated with OBD in combination with all HC adsorber systems is that up to now there has been no means of determining whether the adsorbent of a system is able to efficiently store hydrocarbons without having a method for determining the quantity of HC stored over a period of time under transient operating conditions, the latter requiring a combination of expensive sensors and a complex means of integrating the HC-correlated response.